1. Field of the Invention
The present invention generally relates to test pads, methods, and systems for measuring properties of a wafer. Certain embodiments relate to methods for assessing plasma damage of a wafer. Other embodiments relate to methods and systems for controlling deposition of a charge on a wafer for measurement of one or more electrical properties of the wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, insulating (or dielectric) films may be formed on multiple levels of a substrate using deposition processes such as chemical vapor deposition (“CVD”), physical vapor deposition (“PVD”), and atomic layer deposition (“ALD”). In addition, insulating films may be formed on multiple levels of a substrate using a thermal growth process. For example, a layer of silicon dioxide may be thermally grown on a substrate by heating the substrate to a temperature of greater than about 700° C. in an oxidizing ambient such as O2 or H2O. Such insulating films electrically isolate conductive structures of a semiconductor device formed on the substrate.
Measuring and controlling properties of such insulating films is an important aspect of semiconductor device manufacturing. A number of techniques are presently available for such measurements. For example, electrical measurement techniques, which rely on physical contact between a conductive electrode and an upper surface of an insulating film, are able to determine relevant electrical properties of insulating films using capacitance vs. voltage (“C-V”) and current vs. voltage (“I-V”) measurements. Such measurements have a long history and established utility. However, the necessity of direct physical electrical contact with the insulating film is particularly undesirable in many manufacturing situations.
Non-contacting electrical test techniques have been developed to provide electrical capacitance, electrical thickness, and electrical conductivity information about insulating films. Non-contacting electrical measurements of dielectric properties have a unique advantage of providing electrically derived information without the requirement of physical contact between an electrode and an insulating film. These techniques typically use an ion generation source such as a corona source, and a non-contacting voltage measurement sensor such as a Kelvin Probe or a Monroe Probe to determine electrical properties of the films. Examples of such techniques are illustrated in U.S. Pat. Nos. 5,485,091 to Verkuil, 5,594,247 to Verkuil et al., 5,767,693 to Verkuil, 5,834,941 to Verkuil, 6,060,709 to Verkuil et al., 6,072,320 to Verkuil, 6,091,257 to Verkuil et al., 6,097,196 to Verkuil et al., 6,104,206 to Verkuil, 6,121,783 to Horner et al., 6,191,605 to Miller et al., and 6,202,029 to Verkuil et al., which are incorporated by reference as if fully set forth herein.
Although such techniques are non-contacting, these techniques are often performed on monitor wafers. The term “monitor wafer” is generally used to refer to a wafer that is processed prior to processing one or more product wafers, or a “lot” of product wafers. The term “product wafer” is generally used to refer to a wafer that is processed using a number of semiconductor fabrication processes to form multiple semiconductor devices thereon. In contrast, a monitor wafer is typically only processed in a single semiconductor fabrication process and then recycled or discarded. As such, semiconductor devices are not formed on monitor wafers. Various measurements of the monitor wafer may also be performed prior to processing the lot. The measurements of the monitor wafer may be used to assess the performance of the process. In this manner, the process may be evaluated and potentially altered prior to processing the lot. Therefore, such a method may increase the probability that the process will be within process limits when product wafers are processed.
As the value of semiconductor wafers increases, and the demand for better throughput and more efficient tool usage increases, methods that utilize monitor wafers are becoming increasing undesirable. For example, methods that include processing and measuring a monitor wafer require materials and labor to create and measure the monitor wafer thereby increasing manufacturing costs and reducing throughput. In addition, measurements performed on monitor wafers may not accurately reflect properties of product wafers for a number of reasons. For example, monitor wafers and product wafers may have different characteristics prior to a process that may affect the properties of the wafers after the process. Such characteristics may include, but are not limited to, topography, underlying layers formed on the wafers, and structures formed on the wafers. The characteristics of monitor wafers and product wafers are different because the monitor wafers and the product wafers may be processed differently prior to using the monitor wafers to evaluate a process. For example, the product wafers may be processed using a number of different semiconductor manufacturing processes while a monitor wafer may not be processed using these processes or may be processed using only a subset of these processes. Therefore, a process may not be accurately evaluated, monitored, and controlled using measurements performed on a monitor wafer.
The systems and methods described above may use a corona source in uncontrolled ambient conditions. Corona sources have been used in such uncontrolled ambient conditions for many years and in various processes. For example, corona technology has long been used in xerographic processes such as photocopying and laser printing. The level of corona control required by and used in such applications is only driven by the need to transfer toner particles. This transfer mechanism is relatively insensitive to the level of corona, the species being produced, and the production of byproduct species. In an additional example, electrostatic precipitators commonly use the corona process as part of a pollution control technology. These corona applications are typically relatively large scale in nature, and the industrial applications for which they are designed are also relatively insensitive to the level of corona, the species being produced, and the production of byproduct species. Examples of corona sources are illustrated in U.S. Pat. Nos. 3,495,269 to Mutschler et al., 3,496,352 to Jugle, and 4,734,721 to Boyer et al, which are incorporated by reference as if fully set forth herein.
As described above, non-contacting electrical test methodologies have been developed, which make use of corona sources as a means of depositing charge on the surface of a semiconductor wafer. Examples of corona sources for such methodologies are illustrated in U.S. Pat. Nos. 5,485,091 to Verkuil, 5,594,247 to Verkuil et al., and 5,644,223 to Verkuil, which are incorporated by reference as if fully set forth herein. This application may be commonly referred to as semiconductor metrology. This particular application of corona technology is sensitive to variations in corona deposition and contamination from corona deposition. Corona generation in ambient conditions, however, may produce a number of undesirable byproducts. Such byproducts may include, but are not limited to, ozone, ammonium nitride, and nitric acid. In addition, the electric fields and high voltages may attract particulates into the vicinity of the corona source. In a conventional corona deposition system, such byproducts and particulates may accumulate over time and may deposit onto a wafer and/or change the production of species being created. Therefore, such byproducts and particulates may reduce the accuracy of measurements performed on a semiconductor wafer using a corona source, the accuracy of alterations made to a process using such measurements, and the yield of semiconductor manufacturing processes monitored and altered using a corona source for measurements of electrical properties.
Accordingly, it may be desirable to develop test pads, methods, and systems for measuring properties of a wafer, particularly a patterned wafer, such as plasma damage and electrical properties with relatively high accuracy by reducing the effects of underlying structures on the electrical properties of the structure being measured and/or using controlled deposition of a charge on the wafer.